Niobium-titanium (Nb-Ti) alloys remain the most widely used superconductors, having applications ranging in scale from small laboratory magnets to large scale high energy magnets such as those used in the superconducting supercollider. This leadership position remains despite several alternative materials with higher T.sub.c and B.sub.c2, such as the A-15 compounds (chiefly Nb.sub.3 Sn) and more recently the cuperate oxide superconductors. The main reason for the wide continued use of Nb-Ti is the ease of fabrication of materials which retain technologically interesting and useful T.sub.c, B.sub.c2 and J.sub.c. Most conventional Nb-Ti alloys are designed in the range of 44-50 wt. % Ti. This range contains the peak in H.sub.c2 and T.sub.c. Most attempts to optimize this alloy have been made near the Nb-46.5 wt. % Ti composition. The present best J.sub.ct at 5 Tesla (5T) in a conventional wire is around 3700 A/mm.sup.2. This wire was developed by matching second phase .alpha.-Ti precipitates (which are produced by conventional nucleation and growth techniques) to the fluxon size and spacing. It has been observed that J.sub.ct is proportional to the percent of .alpha.-Ti precipitates. However, 20-25% precipitate is about the maximum which can be developed in this alloy. This apparent limit on volume percent precipitate has currently limited further increases in J.sub.ct.
Various attempts have been made to avoid this limitation by introducing the second phase artificially, and the conductors formed in this manner have been described as artificial pinning center (APC) conductors. Typically, these APC conductors incorporate normal metal rods or tubes in the superconducting matrix of a composite microstructure. Handling considerations generally require that the second phase be quite large. A difficulty is encountered in that the composite then requires a large amount of strain to reduce the second phase to a size comparable to the coherence length, about 5 nm at 4.2K, and therefore strains on the order of 30 are required for some APC conductors versus about 12 to 16 to fully develop conventional Nb-Ti. However, if the initial size of the second phase is on the order of tens of microns, and the ingot diameter is small, the strain needed is only on the order of 16 to 17. Powder metallurgy (PM) can introduce a micron sized second phase in a truly random manner, and allows for greater flexibility in second phase size distribution, volume fraction and chemistry.
However, difficulties are encountered when attempting to produce Nb-Ti alloys using powder metallurgy techniques. First, the starting powders invariably have a higher interstitial content (higher N, O, H and C) than bulk material. These interstitials can harden the alloy to such an extent that it is no longer possible to process the ingot into wire. This limitation exacerbates the basic process of sintering in which sintering efficiency is traded off against reactivity, i.e., processes which produce greater densification generally introduce more interstitials.
A Type II superconductor such as Nb-Ti alloy may carry resistanceless current throughout its bulk and thus can achieve much higher current densities than a Type I superconductor. In a Type II superconductor, the appearance of a voltage across the superconductor is not necessarily an indication that the sample has gone normal, but rather is an indication of the movement of fluxons under the influence of the Lorentz force. Thus, flux pinning becomes an important issue in increasing the current carrying capacity of Type II superconductors. Fluxons arise because the surface energy between the normal and superconducting regions is negative in Type II superconductors in fields above H.sub.c1. A fluxon is characterized by a normal core which is threaded by a quanta of magnetic flux. Thus, fluxons repel each other, establishing a so-called flux line lattice. The magnetic field of the fluxon decays over the length of the penetration depth, and the superconductor is shielded from this magnetic field by circulating supercurrents. These currents are sustained by Cooper pair electrons which interact over the distance of the coherence length--the length over which superconductivity decays. Hence, the radius of the fluxon core is essentially the coherence length, which is about 5 nanometers (nm) at 4.2K.
It has been suggested that because precipitates in Nb-Ti are clustered, the size of the pinning centers (and, in fact, their physical makeup from a superconducting standpoint) may change with temperature due to the proximity effect. An extreme viewpoint is that the larger isolated .alpha.-Ti precipitates pin at lower temperatures while .alpha.-Ti precipitate clusters pin collectively at temperatures near T.sub.c. With conventional Nb-Ti processing, clustering of the pinning centers is to some degree unavoidable.